Scientists are warning there are heightened signs of volcanic activity at Mt Ruapehu and an eruption could be just weeks away.
GNS Science experts say pressure is building up in the crater lake at the volcano and there is now a high risk of an eruption.
Volcanologist Steven Sherburn said: "We think that the temperature a few hundred metres beneath crater lake is about 800C, but the lake itself is only about 20C.
"This suggests the vent is partly blocked, which may be leading to a pressure build-up beneath crater lake. A sudden release of the pressure may lead to an eruption."
Dr Sherburn said the changes in measurements at the volcano over the past few weeks showed that an eruption was more likely over "the next weeks to months".
Although experts were monitoring the activity at the volcano, Dr Sherburn acknowledged that an eruption could come at any time and without warning.
"We are monitoring Ruapehu closely, but it often does not give any immediate warning that it is going to erupt," he said.
As a result, the Aviation Colour Code has increased from green to yellow, indicating a volcano is experiencing signs of elevated unrest above known background levels.
GNS said small earthquakes had been occurring about 5km beneath the summit area of Ruapehu since late October, but these might not be directly related to the high temperatures beneath crater lake because they were much deeper.
There have been several warnings caused by irregular signs of activity at Ruapehu in the past few years.
The last eruption at Mt Ruapehu was in 2007. It was also thought to have been caused by a build-up of pressure beneath crater lake.
A few months earlier a dam holding back the crater lake burst, causing a lahar. In 2006, an eruption also occurred and caused a small earthquake near the crater.